The present invention relates to a process for reducing the level of corrosive impurities in sulfolane which has been used to extract aromatic hydrocarbons from petroleum. Sulfolane (tetrahydrothiophene 1,1-dioxide, also referred to as tetramethylene sulfone), is a colorless highly polar compound having excellent solvent properties, very good chemical and thermal stability and is generally considered to be non-corrosive to carbon steel.
Sulfolane is applied in a number of liquid-liquid and liquid-vapor extractions, and is particularly suitable for the extraction of aromatic compounds such as benzene, toluene and o, m and p-xylenes from hydrocarbon streams having a carbon number from about C.sub.6 to about C.sub.10. These aromatic compounds typically are not plentiful in crude petroleum, but are produced by thermal or catalytic reforming. Many catalytic reforming processes use a platinum-containing catalyst, and up to 1% by weight of a halogen may be used as a promoter to regulate the acidity at the cracking and isomerization sites of the typically alumina support. During catalytic reforming a number of chemical conversions occur, e.g., paraffins may be cracked and hydrogenated, or isomerized, or undergo dehydrocyclization; naphthenes with side chains may undergo dehydrocyclization, and naphthene rings are dehydrogenated into aromatic rings. Thus by selecting feed fractions that are rich in naphthenes it is possible to produce a reformate that contains 35-60% of benzenes, toluene and the xylenes. These aromatic compounds may also be extracted from, e.g., hydrotreated pyrolysis naphtha feeds.
In the course of the extraction process, impurities find their way into the sulfolane solvent, possibly from other processing steps upstream of the extraction process, and possibly from decomposition of the sulfolane, which decomposition may be thermal or catalyzed by the various chemicals present or added to the upstream processes. These impurities can cumulate and concentrate in the sulfolane and lead to significant corrosion of the processing equipment used in the extraction process, and if not remedied, result in serious operating and safety problems. The use of pH control, i.e., maintaining the pH value of the sulfolane above about 8.5 by the addition of nonmetallic acid-neutralizing materials and/or corrosion inhibitors such as amines, particularly aliphatic amines such as methyl ethyl amine, has met with but limited success in alleviating this serious problem. The sulfolane is a comparatively expensive solvent and continual addition of fresh solvent to dilute and to replace contaminated solvent is a costly option.
A procedure has now been found which substantially removes the corrosive impurities from the sulfolane efficiently and with very small loss of the solvent.